The present invention is directed to optical compensation films for liquid crystal displays. More particularly, the invention is directed to optical compensation films formed from polymers that have been irradiated by light.
Compensation films are an indispensable component of modern liquid crystal displays (LCDs). Compensation: films, for example, can compensate the color background of super-twisted nematic LCDs, and can expand the viewing angle of twisted nematic LCDs. The main idea of the optical compensation film (OCF) design is to make the optical symmetry of the compensator more closely resemble that of the director distribution in the liquid crystal layer in the selected state. Therefore, certain parameters of the OCF are determined by the type of LCD mode.
Both negative and positive compensation films, with optics axes oriented in-plane, tilted and normal to the plane of the film, have been developed. In order to adjust more precisely to the director distribution in the liquid crystal (LC) cell, in the modern OCF, the optical axis that is modulated over the film thickness or structure of the OCF is biaxial. Alternatively, a stack of uniaxial films with different directions of optical axis could be used.
Conventional optical compensation films are produced from polymer materials. The OCFs in general use are stretched polycarbonate, yielding negative retardation, and polystyrene, yielding positive retardation, films. These compensators have almost the same wavelength dispersion as that of the liquid crystals and the compensators have high optical anisotropy. The main drawback of stretched films is that it is rather difficult to produce the uniform retardation over a large area and to control the optical axis direction.
Another approach is the use of polymer liquid crystalline films. Cholesteric polymer films are used for the color compensation of LCDs. Discotic compound films expand viewing angle of LCDs. Discotic films possess high negative birefringence and have similar orientation structure to that of the on-state LCD. Uniform alignment of LC layers is usually induced with aligning substrates or external field. The obtained structure is usually fixed by photo-crosslinking or polymerization process. The main problems of the application of liquid crystalline films are the complication of their alignment and high commercial cost.
What is needed in the art is a method of fabricating an optical compensation film wherein the alignment in the film is reached without stretching, application of electric and/or magnetic fields, or special treatment of the aligning substrates, and which can form a uniform retardation over a large area.
It is therefore an object of the invention to provide a method of fabricating an optical compensation film for a liquid crystal display without stretching, application of an electric and/or magnetic field, or special treatment of the aligning substrates.
The present invention provides an optical compensation film for a liquid crystal display comprising a polymer capable of producing light induced anisotropy characterized in that the polymer has been irradiated with light to form at least one optical axis.
The present invention also provides a method of making an optical compensation film comprising: providing a substrate; disposing a polymer in a solvent on the substrate; removing the solvent to form a polymer film; and irradiating the polymer with light, wherein at least one optical axis is formed in the polymer film.
The present invention also provides a method of making a liquid crystal display cell containing an optical compensation film comprising: providing two opposed substrates, disposing the optical compensation film on at least one of the substrates on a surface of the substrate that faces the other substrate, irradiating the optical compensation film with light, disposing electrodes at one of i) on the substrate prior to disposing the optical compensation film, and ii) on the optical compensation film, disposing a liquid crystal between the substrates, and sealing the substrates together, wherein at least one optical axis is formed in the polymer film.